From robotic drive chains to conveyor belts in supply chain operations to the sway of wind turbine towers, position sensing is a critical function in a wide range of applications.It can take many forms, including linear, rotary, angular, absolute, incremental, contact and non-contact sensors.Specialized sensors have been developed that can determine position in three dimensions.Position sensing technologies include potentiometric, inductive, eddy current, capacitive, magnetostrictive, Hall effect, fiber optic, optical and ultrasonic.
This FAQ provides a brief introduction to the various forms of position sensing, then reviews a range of technologies that designers can choose from when implementing a position sensing solution.
Potentiometric position sensors are resistance-based devices that combine a fixed resistive track with a wiper attached to the object whose position needs to be sensed.The movement of the object moves the wiper along the track.The position of the object is measured by using a voltage divider network formed by rails and wipers to measure linear or rotational motion with a fixed DC voltage (Figure 1).Potentiometric sensors are low cost, but generally have low accuracy and repeatability.
Inductive position sensors utilize changes in the properties of the magnetic field induced in the sensor coil.Depending on their architecture, they can measure linear or rotational positions.Linear Variable Differential Transformer (LVDT) position sensors use three coils wrapped around a hollow tube; a primary coil and two secondary coils.The coils are connected in series, and the phase relationship of the secondary coil is 180° out of phase with respect to the primary coil.A ferromagnetic core called the armature is placed inside the tube and connected to the object at the location being measured.An excitation voltage is applied to the primary coil and an electromagnetic force (EMF) is induced in the secondary coil.By measuring the voltage difference between the secondary coils, the relative position of the armature and what it is attached to can be determined.A rotating voltage differential transformer (RVDT) uses the same technique to track rotating position.LVDT and RVDT sensors offer good accuracy, linearity, resolution and high sensitivity.They are frictionless and can be sealed for use in harsh environments.
Eddy current position sensors work with conductive objects.Eddy currents are induced currents that occur in conductive materials in the presence of a changing magnetic field.These currents flow in a closed loop and generate a secondary magnetic field.Eddy current sensors consist of coils and linearization circuits.The alternating current energizes the coil to create the primary magnetic field.When an object approaches or moves away from the coil, its position can be sensed using the interaction of the secondary field produced by eddy currents, which affects the coil’s impedance.As the object gets closer to the coil, the eddy current losses increase and the oscillating voltage becomes smaller (Figure 2).The oscillating voltage is rectified and processed by a linearizer circuit to produce a linear DC output proportional to the distance of the object.
Eddy current devices are rugged, non-contact devices typically used as proximity sensors.They are omnidirectional and can determine the relative distance to the object, but not the direction or absolute distance to the object.
As the name suggests, capacitive position sensors measure changes in capacitance to determine the position of the object being sensed.These non-contact sensors can be used to measure linear or rotational position.They consist of two plates separated by a dielectric material and use one of two methods to detect the position of an object:
In order to cause a change in the dielectric constant, the object whose position is to be detected is attached to the dielectric material.As the dielectric material moves, the effective dielectric constant of the capacitor changes due to the combination of the area of ​​the dielectric material and the dielectric constant of air.Alternatively, the object can be connected to one of the capacitor plates.As the object moves, the plates move closer or farther, and the change in capacitance is used to determine the relative position.
Capacitive sensors can measure displacement, distance, position and thickness of objects.Due to their high signal stability and resolution, capacitive displacement sensors are used in laboratory and industrial environments.For example, capacitive sensors are used to measure film thickness and adhesive applications in automated processes.In industrial machines, they are used to monitor displacement and tool position.
Magnetostriction is a property of ferromagnetic materials that causes the material to change its size or shape when a magnetic field is applied.In a magnetostrictive position sensor, a movable position magnet is attached to the object being measured.It consists of a waveguide consisting of wires that carry current pulses, connected to a sensor located at the end of the waveguide (Figure 3).When a current pulse is sent down the waveguide, a magnetic field is created in the wire that interacts with the axial magnetic field of the permanent magnet (the magnet in the cylinder piston, Figure 3a).The field interaction is caused by twisting (the Wiedemann effect), which strains the wire, producing an acoustic pulse that propagates along the waveguide and is detected by a sensor at the end of the waveguide (Fig. 3b).By measuring the elapsed time between the initiation of the current pulse and the detection of the acoustic pulse, the relative position of the position magnet and therefore the object can be measured (Fig. 3c).
Magnetostrictive position sensors are non-contact sensors used to detect linear position.Waveguides are often housed in stainless steel or aluminum tubes, enabling these sensors to be used in dirty or wet environments.
When a thin, flat conductor is placed in a magnetic field, any current flowing tends to build up on one side of the conductor, creating a potential difference called the Hall voltage.If the current in the conductor is constant, the magnitude of the Hall voltage will reflect the strength of the magnetic field.In a Hall-effect position sensor, the object is connected to a magnet housed in the sensor shaft.As the object moves, the position of the magnet changes relative to the Hall element, resulting in a changing Hall voltage.By measuring the Hall voltage, the position of an object can be determined.There are specialized Hall-effect position sensors that can determine position in three dimensions (Figure 4).Hall-effect position sensors are non-contact devices that provide high reliability and fast sensing, and operate over a wide temperature range.They are used in a range of consumer, industrial, automotive and medical applications.
There are two basic types of fiber optic sensors.In intrinsic fiber optic sensors, the fiber is used as the sensing element.In external fiber optic sensors, fiber optics are combined with another sensor technology to relay the signal to remote electronics for processing.In the case of intrinsic fiber position measurements, a device such as an optical time domain reflectometer can be used to determine the time delay.The wavelength shift can be calculated using an instrument that implements an optical frequency domain reflectometer.Fiber optic sensors are immune to electromagnetic interference, can be designed to operate at high temperatures, and are non-conductive, so they can be used near high pressure or flammable materials.
Another fiber-optic sensing based on fiber Bragg grating (FBG) technology can also be used for position measurement.The FBG acts as a notch filter, reflecting a small fraction of the light centered on the Bragg wavelength (λB) when illuminated by broad-spectrum light.It is fabricated with microstructures etched into the fiber core.FBGs can be used to measure various parameters such as temperature, strain, pressure, tilt, displacement, acceleration and load.
There are two types of optical position sensors, also known as optical encoders.In one case, light is sent to a receiver on the other end of the sensor.In the second type, the emitted light signal is reflected by the monitored object and returned to the light source.Depending on the sensor design, changes in light properties, such as wavelength, intensity, phase or polarization, are used to determine the position of an object.Encoder-based optical position sensors are available for linear and rotary motion.These sensors fall into three main categories; transmissive optical encoders, reflective optical encoders, and interferometric optical encoders.
Ultrasonic position sensors use piezoelectric crystal transducers to emit high-frequency ultrasonic waves.The sensor measures the reflected sound.Ultrasonic sensors can be used as simple proximity sensors, or more complex designs can provide ranging information.Ultrasonic position sensors work with target objects of a variety of materials and surface features, and can detect small objects at greater distances than many other types of position sensors.They are resistant to vibration, ambient noise, infrared radiation and electromagnetic interference.Examples of applications using ultrasonic position sensors include liquid level detection, high-speed counting of objects, robotic navigation systems, and automotive sensing.A typical automotive ultrasonic sensor consists of a plastic housing, a piezoelectric transducer with an additional membrane, and a printed circuit board with electronic circuits and microcontrollers for transmitting, receiving, and processing signals (Figure 5).
Position sensors can measure absolute or relative linear, rotational and angular motion of objects.Position sensors can measure the movement of devices such as actuators or motors.They are also used in mobile platforms such as robots and cars.A variety of technologies are used in position sensors with various combinations of environmental durability, cost, accuracy, repeatability, and other attributes.
3D Magnetic Position Sensors, Allegro MicrosystemsAnalyzing and Enhancing the Security of Ultrasonic Sensors for Autonomous Vehicles, IEEE Internet of Things Journal How to select a position sensor, Cambridge Integrated CircuitsPosition sensor types, Ixthus InstrumentationWhat is an inductive position sensor?, Keyence What is Magnetostrictive Position Sensing?, AMETEK
Browse the latest issues of Design World and back issues in an easy-to-use, high-quality format.Edit, share and download today with the leading design engineering magazine.
The world’s top problem-solving EE forum covering microcontrollers, DSP, networking, analog and digital design, RF, power electronics, PCB routing, and more
Copyright © 2022 WTWH Media LLC.all rights reserved.The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used without the prior written permission of WTWH MediaPrivacy Policy |Advertising | About Us